Feeling weird about something my boyfriend did

So, my boyfriend and I have known each other since November 2018. We started seeing each other the same month but agreed we wouldn't be exclusive. He'd just gotten out of a bad relationship and I didn't really know what I wanted.

He said I love you in June 2019. I moved in with him soon after. To my understanding, neither of us had been with anyone else during our time of inclusivity. I didn't have sex with anyone, and although I'd asked him before if he had relationships, he said no.

I asked him tonight while watching Netflix, and this time I specified having sex with someone as a part of something I want to know. He said he did. He had sex with a girl in April 2019. We got our current dog a month and a half later.

I had no clue this happened. And I'm quite frankly confused. I feel anxious, betrayed, overwhelmed and sad. I know this happened a year ago but it feels weird to just find out now. I was in love with him at this point in our relationship, but he was having sex with others.